(11)5.1 Introduction
5.2 Scintillators and scintillation detectors
5.3 Pulse processing
5.4 Coincidence processing
5.5 Dead-time
5.6 Block detectors
5.7 Camera configurations in PET
Detection systems are a key component of any
imaging system, and an understanding of their properties is important
for establishing appropriate operating criteria or designing schemes
for obtaining quantitative information. In this section scintillation
detection systems, which are used in the majority of PET tomographs,
are discussed.
5.2 Scintillators and scintillation detectors
The scintillation process involves the conversion of high-energy photons into visible light via interaction with a scintillating material, and consists of the following steps:
1) a photon incident on the scintillator creates an energetic electron, either by Compton scatter or by photoelectric absorption.
2) as the electron passes through the scintillator, it loses energy and excites other electrons in the process.
3) these excited electrons decay back to their ground state, giving off light as they do so.
In a scintillation detector, the scintillator is optically coupled to a photomultiplier tube (PMT) which then generates an electrical signal in response to light incident upon its face. There are several variations on this theme which are used in PET - for example, a 2D array of crystals may be coupled to 4 PMTs (the block-detector, Casey and Nutt, 1986), an array of PMTs may be coupled to a single planar crystal (the Anger camera,Anger, 1958), or an array of crystals may be coupled to a multi-channel PMT (Cherry et al 1997).
Compton scatter and photo-electric absorption generate electrons of differing energy distributions. In photo-electric absorption, all the photon energy is transferred to the electron, and the energy distribution of the photo-electrons is sharply peaked close to the energy of the incident photon. In Compton scatter, the recoil electrons have a range of energies, depending on the scattering angle. From equation 1 we can say that the energy of the recoil electron will be
(11)
where q is the photon scattering angle, Ebis the energy of the electron, E is the energy of the incident photon and Eí is the energy of the scattered photon. Ebreaches a maximum when q = p . For 511 keV photons, this value is 340.7 keV. A typical energy distribution for electrons involved in interactions with 511 keV photons is shown in figure 17. When real scintillation detectors are exposed to mono-energetic photons, the energy measured is not that of the electron generated by the initial interaction, but rather the total energy deposited by the photon in the detector. This distinction is important because photons initially interacting by Compton scatter may subsequently be involved in further interactions within the detector. In a sufficiently large detector, most Compton-scattered photons will eventually deposit all their energy, and most events will register in the photon energy peak. Under these circumstances, this feature of the energy distribution is better described by the term "full-energy peak" rather than "photo-peak".
Figure 17. Features of a typical energy distribution for
electrons involved in interactions with 511 keV photons.
In small detectors, photons may escape after
depositing only part of their energy in the crystal, and the measured
energy distribution is closer to that shown in figure18.
In practice, the energy distribution is also blurred by the finite
energy resolution of the detector system, and by the fact that the
incident radiation is not mono-energetic, as a proportion of the
photons will have undergone Compton scatter prior to detection. There
will also be some events with energy greater than the full-energy
peak, where photon interactions with the detector occur sufficiently
close together in time that they cannot be resolved as separate
events. The energy resolution of the system is defined as the ratio
of the full-width at half-maximum (FWHM) of the full energy peak and
the energy value at the full energy peak maximum.
Figure 18. Features of a typical
energy distribution measured by a scintillation detector system
exposed to 511 keV photons.
If a large proportion of the incident photons register in the full-energy peak and the energy resolution of the detector system is good, then it is possible to discriminate against events arising from photons scattered within the object by rejecting those with a low measured energy. As discussed above, the proportion of events in the full-energy peak increases with increasing detector size. However, large detectors reduce the spatial resolution of the system. The number of events in the full-energy peak can also be increased by increasing the proportion of photons which interact by photo-electric absorption. This may be achieved by choosing scintillators with a large value of effective atomic number (effective Z). The linear attenuation coefficient also increases with increasing effective Z, so choosing scintillators with high effective Z also increases sensitivity. The energy resolution of the system may be improved by improving the statistical quality of the signal from the PMT - this may be achieved by increasing the number of scintillation photons incident upon its face. In summary, the following qualities are desirable in a scintillator:
PET detectors have to work at high count-rates, so it is important that the decay time of the scintillator should be short. Shorter decay times also allow faster timing signals for coincidence detection. An undesirable characteristic in a scintillator is the existence of secondary scintillation components with long decay times, as in high count-rate operation these can cause a build-up of back-ground light.
The final considerations are that the scintillator should ideally be robust and easy to manufacture.
The properties of some important scintillators are
shown in table 1.4. Some of these, e.g. NaI and LSO, require doping
with an activator substance in order to obtain optimum scintillation
properties. NaI has been used extensively in SPECT and to a lesser
extent in PET, although it is both hygroscopic and fragile. However
the scintillator of choice for PET cameras using block-detectors has
been Bismuth germanate (BGO), which has a high effective Z, is not
hygroscopic and does not have long-lived secondary scintillation
components. Cerium-doped lutetium oxy-orthosilicate or LSO
(Daghighian,
et al 1993) is a scintillator which
shows considerable promise for the future, and has already been used
with some success (Cherry
et al 1997). However, it is
currently significantly more difficult to manufacture than
BGO.
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Effective atomic no. (Z) |
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Lin. atten. coef. (cm-1) |
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Index of refraction |
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Light yield [%NaI:Tl] |
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Peak Wavelength (nm) |
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Decay const. (nS) |
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Fragile |
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Hygroscopic |
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Table 4. Examples of scintillators and their properties. Compiled
from Dahlbom
et al (1997), Ficke
et al (1996), Knoll
(1989), Phelps
et al (1986) and Bailey
(1996).
When a scintillation detector detects a photon, the electrical pulse generated by the PMT or PMT array is used to generate a timing signal. This is done by passing the pulse through a constant fraction discriminator (CFD), which generates a digital pulse when the signal reaches a constant fraction of the peak pulse height. This pulse is then used in the coincidence circuitry.
The pulses from the PMTs are also passed through a
differential discriminator to sort them according to pulse height.
Usually there is a lower energy-level discriminator (LLD), and an
upper energy-level discriminator (ULD) which may be used to reject
pulses below or above particular values. The LLD can be used to
discriminate against scatter, as scattered annihilation photons have
lower energy than those which are unscattered. Not all scatter can be
removed this way, as many scattered photons have an energy quite
close to 511 keV and the energy resolution of typical detector
systems is insufficient to distinguish them from unscattered photons.
Also, as discussed in section
5.2 a significant proportion of
unscattered photons interact with the scintillator by the Compton
process, and many of these will not deposit all their energy in the
crystal. Therefore a high value for the LLD will cause the rejection
of a significant number of true events and there is a trade-off
between scatter fraction and sensitivity to true coincidences.
Optimisation of LLD value is discussed in detail by Badawi
et al (1996). The ULD may be used
to reject some events where more than one photon is incident on the
block-detector at the same time. LLDs and ULDs have also been used to
divide the acquired data into different energy-windows for analysis
(e.g. Shao
et al 1994, Grootoonk
et al 1996).
Once timing signals have been generated from each
pulse, they are passed to coincidence circuitry for
processing. Figure
19 shows a schematic diagram of a
coincidence circuit. There will usually be some time difference
between two timing pulses arising from a coincidence event due to the
finite time resolution of the detector and CFD system. In order to
deal with this, the timing pulses are passed to a gate generator,
which creates an electronic pulse of duration t . t is
known as the coincidence resolving time of the system. These
fixed-width pulses are then passed to a logic unit, which generates a
pulse if there is a signal on both of its inputs at the same time. So
if a timing pulse is generated on one channel at time t, a
coincidence will be recorded if there is a timing pulse on the other
channel between t - t and t + t . The value of t must be carefully chosen. If it is too small compared to the time
resolution of the detection system, true coincidences will be missed.
If it is too large, more random coincidences will be counted without
significant increase in the number of true coincidences. For a
typical BGO block-detector system t is about 12 nS - considerably shorter than the decay time for BGO,
which is about 300 nS.
Figure 19 Schematic diagram showing coincidence processing in a PET camera.
If t is sufficiently small, then time-of-flight effects may become
important. These effects occur when the time taken for one photon
from an annihilation event takes significantly longer to reach the
detectors than the other. For a camera with a ring diameter of 1m and
an active FOV of 60 cm, the maximum time difference for photons to
reach the detectors is about 2.7 nS. Some attempts have been made to
build PET imaging systems which make use of time-of-flight
information to improve spatial resolution and signal-to-noise ratio
(Ter
Pogossian et al 1981). These
require extremely fast scintillators such as BaF2, which
has a fast-component decay constant of 0.8 nS.
All detection systems have limits to the rate at which events may be processed. For example, if light pulses from separate photon interactions overlap to a significant extent, only one pulse will be measured by the PMTs. This is known as pulse pile-up. The electronics will also have a finite maximum rate at which they can process data, with typical maximum rates for pulse-processing electronics being around 1 MHz. These facts mean that some events will be missed. Since nuclear decay is a random process, there will always be a finite probability that some events will occur too close together to be distinguished even at very low average count-rates. At high count-rates such losses can become very significant. These losses are known as dead-time losses.
In practice, dead-time losses tend to be dominated by pile-up within the scintillation crystals. As a result, cameras with larger numbers of discrete scintillation detectors (such as those employing block-detector technology, which may have several hundred) can operate at higher count-rates than those with fewer (such as DHCI systems which have only two large area detectors).
In a block detector, a 2D array of crystals are attached to 4 PMTs (see figure 20) via a light guide. Usually the array will be cut from a single crystal and the cuts filled with light-reflecting material. When a photon is incident on one of the crystals, the resultant light is shared by all 4 PMTs. Information on the position of the detecting crystal may be obtained from the PMT outputs by calculating the following ratios and comparing them to pre-set values:
(12)
(13)
where A, B, C and D are the fractional amounts of light detected by each PMT (Casey, 1992). The positioning algorithm is subject to statistical error, the magnitude of which depends on the number of scintillation photons detected by the PMTs.
At high count-rates, light-pulses from two or more
photon interactions may overlap as described in section
5.5 above. If the photons are incident on
different crystals, the resultant pulse may then be incorrectly
assigned to a crystal lying between them. Event mis-positioning can
also occur when photons incident in one crystal penetrate another
prior to interaction, or when photons are scattered in one crystal
and subsequently interact with another. The latter two categories of
mis-positioning are common to many types of detection
systems.
Figure 20. A block detector
5.7 Camera configurations in PET
A pre-requisite for high-resolution images in PET is accurate localisation of detected annihilation photons. This may be achieved by using small detectors. Early PET cameras used small individual scintillating crystals attached to single PMTs arranged in a ring. The main limiting factor on scintillation detector size is the PMT, and using this configuration it is extremely difficult to construct cameras with an axial extent greater than one or two rings of detectors. This constraint led to the development of the block detector described above. Cameras with up to 48 adjacent rings of crystal detector elements and an axial FOV of ~24 cm have been constructed using block-detector technology (Jones et al 1996). Cheaper versions of full ring cameras have been built which consist of a partial ring of block detectors which rotate around the FOV (Bailey et al 1997). A recent solution to the problem of detector packing has been to use small crystals with a high light output (made from LSO) coupled to optical fibres, which guide the light to a multi-channel PMT some distance away (Cherry et al 1997). Another approach is to use pairs of large-area planar detectors. To achieve acceptable spatial resolution with large-area planar detectors a high scintillation light output is required, and NaI is the scintillator of choice in such systems. These systems are considerably cheaper to manufacture than full-ring PET cameras, although they do suffer from several intrinsic performance problems (Jarritt and Acton 1996). A more effective variation on this theme is to use 6 pl
anar detectors arranged in a hexagon (Muehllehner and Karp, 1986).
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Revision date:
12 Jan 1999